An antibody-drug conjugate is a new form of drug, in particular used as an anticancer agent with targeted action. The therapeutic antibody binds to one or more cell types as a function of its specificity and conventionally releases a cytotoxic agent in the cells.
An antibody-drug conjugate constitutes a means for selective delivery of a cytotoxic agent. This engineering therefore makes it possible to combine the specificity of targeting by antibodies with powerful new effector functions by the agents with which they are conjugated.
To date, more than 20 antibody-drug conjugates are under development (Senter, P. D. et al. Annu. Rev. Med. 2013, 64, 15-29).
The general structure of an antibody-drug conjugate is as described in FIG. 1.
The product linking the antibody and the drug is called a linking agent or linker. It can be grafted onto the antibody via at least one of the eight cysteines forming the 4 interchain disulphide bridges or onto at least one of the eight lysines.
The number of drug molecules grafted onto the antibody determines a ratio called the Drug-Antibody Ratio (DAR).
After binding to its target antigen, the antibody is internalized in the cell by receptor-mediated endocytosis. The vesicles fuse with lysosomes where the cytotoxic molecule is released from the antibody by various mechanisms. The active cytotoxic agent then acts directly on the cell, inducing its death, and sometimes on the neighbouring cancer cells by transport or diffusion in the environment.
The antibody is therefore used mainly as a vector and delivers the cytotoxic agent into the cell.
By “cytotoxic agent” is meant a molecule capable of inhibiting or preventing the function of a cell.
The cytotoxic agent bound to the antibody is a prodrug that becomes active in the tumour cell after release (Jaracz et al. 2005). The activity of the free molecule must be sufficient to kill the cancer cell, even at low concentration.
Despite their growing success, antibody-drug conjugates have a notable drawback, because they are more complex and have a heterogeneous structure compared to the corresponding original antibody. Attachment of a cytotoxic agent via a linker has a significant influence on the pharmacokinetics and pharmacodynamics of the antibody (PK-PD) and therefore on the therapeutic index.
The antibody-drug conjugates preferably have an average DAR equal to 4 for optimum activity but this ratio can vary from 1 to 13, knowing that for a DAR there can be a population of distinct entities.
Many researchers are working on this heterogeneity. Tests have been carried out with the aim of modifying the sequence of the antibody, by introducing natural or non-natural amino acids (Mallet, W. et al. Nat. Biotechnol. 2008, 26, 925-932 and Schultz, P. G. et al. Proc. Natl. Acad. Sci. 2012, 109, 16101-16106). Nevertheless, this method has a disadvantage, that of having to be transposed for each antibody of interest, which requires consistent work on the starting antibody.
Consequently, the technical problem that arises from the prior art is to obtain a technological means allowing controlled grafting of a cytotoxic agent without modifying the sequence of the antibody of interest—a technological means that can be adapted to any antibody.